Re: [RFC V2 12/21] rv/reactor: Add the printk reactor

[Shuah Khan <skhan@linuxfoundation.org>]

On 2/14/22 3:45 AM, Daniel Bristot de Oliveira wrote:
> Sample reactor that printks the reaction message.
> 

Who is supposed to take action on this message and how do they
get notified?

> Note: do not use this reactor with rq_lock taken, it will lock the
> system until printk can handle that.

Please give more details on "lock the system" and how locking
would impact users and system operation

> 
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Marco Elver <elver@google.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Shuah Khan <skhan@linuxfoundation.org>
> Cc: Gabriele Paoloni <gpaoloni@redhat.com>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Clark Williams <williams@redhat.com>
> Cc: linux-doc@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-trace-devel@vger.kernel.org
> Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
> ---
>   kernel/trace/rv/Kconfig          |  8 ++++++
>   kernel/trace/rv/Makefile         |  3 ++-
>   kernel/trace/rv/reactor_printk.c | 43 ++++++++++++++++++++++++++++++++
>   3 files changed, 53 insertions(+), 1 deletion(-)
>   create mode 100644 kernel/trace/rv/reactor_printk.c
> 
> diff --git a/kernel/trace/rv/Kconfig b/kernel/trace/rv/Kconfig
> index f2d0bed05a55..490f1417def9 100644
> --- a/kernel/trace/rv/Kconfig
> +++ b/kernel/trace/rv/Kconfig
> @@ -37,4 +37,12 @@ config RV_REACTORS
>   	  tracing reactions, printing the monitor output via tracepoints,
>   	  but other reactions can be added (on-demand) via this interface.
>   
> +config RV_REACT_PRINTK
> +	tristate "Printk reactor"
> +	depends on RV_REACTORS
> +	default y if RV_REACTORS
> +	help
> +	  Enables the printk reactor. The printk reactor emmits a printk()
> +	  message if an exception is found.
> +
>   endif # RV
> diff --git a/kernel/trace/rv/Makefile b/kernel/trace/rv/Makefile
> index edad01bb2b5d..d86ceb103ef2 100644
> --- a/kernel/trace/rv/Makefile
> +++ b/kernel/trace/rv/Makefile
> @@ -1,6 +1,7 @@
>   # SPDX-License-Identifier: GPL-2.0
>   
>   obj-$(CONFIG_RV) += rv.o
> -obj-$(CONFIG_RV_REACTORS) += rv_reactors.o
>   obj-$(CONFIG_RV_MON_WIP) += monitor_wip/wip.o
>   obj-$(CONFIG_RV_MON_WWNR) += monitor_wwnr/wwnr.o
> +obj-$(CONFIG_RV_REACTORS) += rv_reactors.o
> +obj-$(CONFIG_RV_REACT_PRINTK) += reactor_printk.o
> diff --git a/kernel/trace/rv/reactor_printk.c b/kernel/trace/rv/reactor_printk.c
> new file mode 100644
> index 000000000000..9f9c9bb2d304
> --- /dev/null
> +++ b/kernel/trace/rv/reactor_printk.c
> @@ -0,0 +1,43 @@
> +/*
> + * Printk RV reactor:
> + *   Prints the exception msg to the kernel message log.
> + *
> + * Copyright (C) 2019-2022 Daniel Bristot de Oliveira <bristot@kernel.org>
> + *
> + * SPDX-License-Identifier: GPL-2.0
> + */
> +#include <linux/ftrace.h>
> +#include <linux/tracepoint.h>
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/init.h>
> +#include <linux/rv.h>
> +
> +static void rv_printk_reaction(char *msg)
> +{
> +	printk(msg);
> +}
> +
> +struct rv_reactor rv_printk = {
> +	.name = "printk",
> +	.description = "prints the exception msg to the kernel message log",
> +	.react = rv_printk_reaction
> +};
> +
> +int register_react_printk(void)
> +{
> +	rv_register_reactor(&rv_printk);
> +	return 0;
> +}
> +
> +void unregister_react_printk(void)
> +{
> +	rv_unregister_reactor(&rv_printk);
> +}
> +
> +module_init(register_react_printk);
> +module_exit(unregister_react_printk);
> +
> +MODULE_LICENSE("GPL v2");
> +MODULE_AUTHOR("Daniel Bristot de Oliveira");
> +MODULE_DESCRIPTION("printk rv reactor: printk if an exception is hit");
>